1. Field of the Invention
The present invention relates to an improvement of a control apparatus for a drive system of a hybrid vehicle.
2. Description of Related Art
There is known a hybrid vehicle drive system including: a differential device which comprises a first differential mechanism and a second differential mechanism and which comprises four rotary components; an engine, a first electric motor, a second electric motor and an output rotary member which are respectively connected to the four rotary components; and a plurality of coupling elements. JP-2011-98712 A1 discloses an example of a hybrid vehicle transmission system configured to switch the hybrid vehicle drive system to a selected one of a plurality of vehicle drive modes, according to a selected one of different combinations of operating states of the coupling elements.
In the prior art hybrid vehicle drive system described above, it is not possible to maintain an EV drive mode in which the hybrid vehicle is driven by a drive force generated by at least one of the first and second electric motors while an output shaft of the engine is connected to a stationary member, if an operating speed of the first or second electric motor has been raised to an upper limit, even when a sufficiently large amount of an electric energy is stored in a battery. In this case, the electric energy stored in the battery cannot be sufficiently utilized. This problem was first discovered by the present inventor in the process of an intensive study in an effort to improve the performance of the hybrid vehicle.